Although the refrigerant compressor of a vehicular cooling system may be driven by an auxiliary engine or an electric motor, it is more common practice to drive the compressor with the same engine which provides motive power for the vehicle. The drive shaft of the compressor is typically coupled to the engine through an electrically actuated clutch. The clutch facilitates energizing the compressor when it is required for cooling the interior of the vehicle to maintain it at a comfort temperature.
Experience has shown that the compressor clutch may be subject to rapid wear, unless preventive measures are taken. For example, wear on the compressor clutch is reduced if it is energized only when the vehicle engine is at idle speed. Further reduction of clutch wear may be achieved by minimizing the frequency at which the clutch is engaged, i.e., by allowing the compressor to continue to run after the demand for cooling has been met. This reduces compressor cycling, thereby also extending the operating life of the compressor. Of course, once the interior of the vehicle is cooled to a setpoint temperature, continued cooling of the discharge air is undesirable unless it can be reheated before being discharged into the comfort zone of the vehicle. This is generally accomplished by circulating the air through a fluid-to-air heat exchanger, in heat transfer with the vehicle's engine coolant fluid. The same heat exchanger used for reheat is normally used to heat air discharged into the comfort zone during operation of the temperature conditioning system in a heating mode.
Because a vehicle is subjected to rapidly changing environmental conditions, a vehicular temperature conditioning system must respond quickly to changes in the temperature conditioning demand. Conventional controls for such systems are responsive to temperature sensors disposed either in the comfort zone or in the return air duct. As a result, it is not uncommon for the temperature in the comfort zone to fluctuate over a rather wide range, especially during operation in the cooling mode with reheat energized. This fluctuation results from inadequacies of the prior art controls in responding to temperature conditioning demand. Specifically, at termination of the reheat cycle, the reheat heat exchanger disposed in the chilled air stream continues to add heat to the cooled air after the reheat demand is met and after the control has stopped the flow of coolant fluid through the heat exchanger. It is the thermal inertia of the heater due to the hot coolant fluid trapped therein which causes the comfort zone temperature to overshoot. Prior art control systems, whether of the electrical type with "anticipation" means or of the pneumatic proportional type, therefore tend to allow the comfort zone temperature to fluctuate over too wide a range during the cooling cycle, because such controls terminate reheat in response to the return air (or comfort zone) temperature and do not adequately allow for the thermal inertia of the reheat heat exchanger.
Comfort zone temperature overshoot during the cooling mode is especially undesirable. It causes greater discomfort to passengers of the vehicle than does an equivalent overshoot occuring during the heating mode, for the following reason. The cooling mode is initiated when the vehicle is not radiating sufficient heat through its exterior surface nor sufficiently cooled by ventilation air flow to maintain the comfort zone at a setpoint temperature. After the setpoint temperature is attained, reheat is initiated to prevent overcooling of the comfort zone. When the setpoint temperature is exceeded due to reheat overshoot, the vehicle interior remains uncomfortably warm until the refrigerant cooling system reduces the temperature to the comfort level. By comparison, if the overshoot occurs during the heating mode, the windows and exterior surfaces of the vehicle tend to radiate the excess heat to the cold outside ambient air before the passengers become uncomfortably warm.
In consideration of the problems described above, it is an object of this invention to control a vehicular temperature conditioning system in a manner which minimizes reheat overshoot while the system is operating in the cooling mode.
It is further an object of this invention to provide method and control apparatus for such a system which reduces the deviation of the comfort zone temperature from a setpoint while operating in either a heating or cooling mode.
A still further object of this invention is to reduce the wear on a vehicular refrigerant vapor compressor and its associated clutch, while maintaining the comfort zone at a setpoint temperature.
These and other objects of the subject invention will become apparent from the drawings and the description which follows.